This invention relates to processes for growing large single crystal semiconductors and, more particularly, to processes for growing large single crystals of Group III-V semiconductor compounds.
One of the most significant developments in recent years in semiconductor technology has been the increased importance of compound semiconductors. Particularly significant are the Group III-V semiconductor compounds composed of elements of Groups III and V of the periodic table, such as gallium arsenide (GaAs) and indium phosphide (InP). Compound semiconductors are used in such devices as semiconductor lasers, light emitting diodes, microwave oscillators and amplifiers, high speed transistors, and various types of radiation detectors including infrared and visible light detectors. Group III-V semiconductors, particularly GaAs, are increasingly being used for memory and logic integrated circuits, because their higher electron drift velocities make possible faster speeds than comparable conventional devices of silicon.
Most such commercial use requires the growth of large single crystals of the semiconductor. Various methods have been proposed for growing large single-crystal ingots from which wafers can be cut for the subsequent fabrication of useful devices. One of the more promising methods for such crystal growth is the vertical gradient freeze (VGF) method, particularly the VGF method defined in the U.S. patent of W. A. Gault No. 4,404,172. According to this method, polycrystalline starting material is placed in a vertically extending crucible including a small cylindrical seed well portion at its bottom end which snugly contains a seed crystal. Initially, the starting material and a portion of the seed are melted. The power to the system is then reduced in such a manner that freezing proceeds vertically upwardly from the seed crystal. The major advantage of the VGF method is that crystals with very low dislocation densities can be produced using low thermal gradients and slow rates of cooling. One possible drawback to VGF growth is that the interaction of the melt with the crucible may lead to the introduction of dislocations or cause false grains to nucleate and spoil the single crystal.
It is well known that the III-V compounds tend to dissociate at higher temperatures, with the more volatile Group V element escaping into the vapor phase. Several approaches have been developed to prevent or retard this tendency. For example, in one approach to the growth of gallium arsenide, the more volatile arsenic component is prevented from escaping by providing a vapor pressure of arsenic vapor over the melt from a separately heated reservoir of arsenic within the sealed growth container. It is also known in the art that arsenic loss from the melt may be retarded with the use of any of various materials such as boric oxide (B.sub.2 O.sub.3), barium chloride (BaCl.sub.2), or calcium chloride (CaCl.sub.2) which act as diffusion barriers. Such additives, having a lower density than the molten gallium arsenide, rise to the surface, encapsulate the melt, and, together with an inert gas pressure in the vessel, can contain the volatile arsenic vapors; see, for example, the paper "Growth of Single Crystals of GaAs in Bulk and Thin Film Form," by B. A. Joyce, included in the book "Crystal Growth," edited by B. R. Pamplin, Pergamon Press, 1975, pp. 157-184 at page 165.
Various attempts have been made over the years to grow III-V crystals by the VGF technique utilizing different crucible materials. See for example, the aforementioned Pamplin book at pp. 389-391. A significant limitation on the success of these efforts has been the physical and chemical interaction of the melt with the crucible wall. See, for example the aforementioned Pamplin book, p. 389, and "The Art and Science of Growing Crystals," J. J. Gilman, Ed., John Wiley & Sons, New York, 1963, at p. 366 and p. 390. It has proved difficult to reproducibly decouple this interaction in order to eliminate spurious polycrystalline growth that results.